The field of the invention is polyamide powders for the coating of substrates and the invention is particularly concerned with pulverulent polyamide coating compositions based on polyamides having an average of at least nine carbon atoms per carbonamide group for use in fusion coating procedures.
The state of the prior art of producing polyamide powders may be ascertained by reference to U.S. Pat. Nos. 2,698,966; 2,742,440; 2,975,128; 3,203,822; 3,299,009; 3,476,711; 3,927,141; and 3,966,838; British Patent Nos. 535,138 and 830,757; West German Published application Nos. 1,570,392; 1,669,821; 1,210,183; 1,267,428; 1,520,551 and 1,495,147; East German Patent Nos. 7283; 23,607; 24,307 and 25,173; French Patent Nos. 951,924 and 1,384,988 and Swiss Patent No. 582,517; and the Kirk-Othmer "Encyclopedia of Chemical Technology" 2nd Ed., Vol. 16 (1968), under the section "Polyamide (Plastics)", pages 88-105, particularly page 92--polylauryllactam (nylon-12), and polyundecanamide (nylon-11), page 101 Solution Processes, and Powder Processing, pages 101-102, the disclosures of which are incorporated herein.
The flame spraying and fluidized bed coating of nylon on a metal base is disclosed in U.S. Pat. No. 3,203,822. U.S. Pat. No. 3,410,832 discloses the preparation of polymers and copolymers of lauryllactam
It is also known from the disclosures of Chem. Ind. 1968, pp 783 and Modern Plastics, February 1966, pp 153-156, that polyamide powders which are based upon polyamides having at least 11 carbon atoms are useful in the production of metallic coatings similar to enamels.
The compositions of the present invention are applied by the fusion coating method, that is by the fluidized bed method, the flame spray method or the electrostatic coating method. The polyamide powders are obtained by precipitating the polyamide from solutions as disclosed in British Patent No. 688,771 or U.S. Pat. No. 4,334,056, or by grinding the polyamide granulates as disclosed in U.S. Pat. Nos. 4,143,025 and 4,195,162.
A variety of approaches for improving the properties of polyamide based coating compositions are known from the disclosures of German Patent No. 1,271,286; U.S. Pat. Nos. 3,900,607; 3,927,141; and 4,273,919; and British Patent No. 1,392,949.
Nevertheless the coating powders which are known still require further improvements. With particular regard to the edges of very small radius of curvature, that is for thin wires and especially for molded articles made from such wires, for instance baskets, irregularities take place especially at the crossing sites, depending on the position and the stress placed on the coating.
Thus, pore free coatings are achieved at the crossing points of wire grids using polyamide powders having low molecular weights (small relative spec. viscosity values), however, the edges of the wire ends are only inadequately coated. Powders of higher molecular weights are not helpful because pore formation increases substantially and the required elasticity is no longer retained.